Frontbencher Joel Fitzgibbon has fired a salvo in one of the most electorally important battles for Bill Shorten at the coming ALP national conference – whether a Labor government should turn back boats.

Fitzgibbon, a former defence minister, predicted that turnbacks would be part of Labor’s policy for the election. “Let’s have the debate at national conference. I believe that will be the outcome,” he said.

Fitzgibbon’s view represents that of the NSW right, of which he is a member, but the issue is difficult for Shorten and the conference.

It is one about which many ALP members feel passionately; the national conference make-up between right and left is closer and more uncertain than usual; and Labor at the last election was highly critical of the Coalition’s policy.

Shorten is already in a tricky position. When immigration spokesman Richard Marles last year signalled Labor might embrace turnbacks, Shorten slapped him down, saying “the case has not been made out for change”.

But there is a growing feeling in the parliamentary party that Labor needs to alter its policy.

If the conference said an ALP government should not turn back boats, it would be handing the Liberals a big weapon for the election. Together with the tough offshore processing regime, turnbacks have been regarded as important in stopping the people smuggling trade.

The parliamentary party is, in theory, bound by what conference decides, although in practice the MPs have exercised considerable flexibility.

The draft platform for the July conference is silent on turnbacks.

The Labor for Refugees group is gearing up for a strong fight at the conference.

Its national co-convenor Robin Rothfield told The Conversation on Sunday that an amendment on turnbacks was being prepared based on ACTU policy.

Rothfield said the proposed amendment would go to a meeting of the national left in Sydney at the coming weekend.

It reads: “Labor rejects other policies of ‘deterrence’ implemented alongside offshore detention, especially intercepting and turning back boats at sea, or transferring refugees to other vessels for immediate return to their countries of origin without a proper assessment of their claims for protection.

“Such policies needlessly put both asylum seekers and seafarers in danger. Provisions in the Migration and Maritime Powers Legislation Amendment Act 2014 which facilitate boat turnbacks and give the Immigration Minister the power to secretly suspend the application of Australian Maritime Law and International Maritime Conventions to any vessel must be repealed.”

Fitzgibbon told Sky a range of tools was needed to ensure the flow of boats did not resume and “one of those tools currently is boat turnbacks”.

“Personally I can’t see that there’s an overwhelming argument that turnbacks isn’t an important part of the tool kit.” Fitzgibbon said there was a universal commitment within shadow cabinet to ensuring the asylum seeker flows did not begin again.

The ALP’s incoming national president, Mark Butler, from the left, said that Labor was “committed to making sure the boat passageway between Java and Australia remains closed”.

Pressed on whether he thought that turnbacks should be part of Labor’s policy, Butler told reporters that one of the concerns Australians had with this policy area was “the government’s obsession with secrecy, particularly their obsession with secrecy they have around the turnbacks operations they have in place.

“Particularly around questions involving safety at sea, for everyone involved including Navy personnel, but also the impact on relations with our important neighbour, Indonesia.”

The government is already preparing its counter if Labor does say it will turn back boats. Immigration Minister Peter Dutton was out on Sunday declaring that Labor in government would not follow through with action.

Postscript: Mirabella on the march

Former Liberal frontbencher Sophie Mirabella on Sunday won preselection for her old seat of Indi, making the Victorian electorate one of the most interesting contests to come. In 2013, Mirabella lost what had been a safe Liberal seat to independent Cathy McGowan, who ran a campaign based on localism.

Mirabella, who would have been a cabinet minister in the Abbott government, had ignored the signs over years of an eroding vote. Bidding for preselection, she admitted she had spent too much time away from her home patch. “Clearly, I got the balance wrong,” she wrote to preselectors. But some Liberals believe she could be a drag on the party’s vote because of her previous record.

The Nationals have indicated they will also run a candidate. Their strategy has been to position themselves, if McGowan held the seat, to mount a strong bid for it on her likely retirement after another term.